1. Technical Field
The present invention relates to a liquid ejecting apparatus equipped with a liquid ejecting head such as an ink jet printing head, and a control method thereof, and particularly, relates to a liquid ejecting apparatus ejecting a liquid inside a pressure chamber from a nozzle by applying a variation in the pressure to the pressure chamber communicating with the nozzle, and a control method thereof.
2. Related Art
A liquid ejecting apparatus is an apparatus that includes a liquid ejecting head capable of ejecting a liquid and ejects various types of liquids from the liquid ejecting head. As a typical example of the liquid ejecting apparatus, for example, an image forming apparatus such as an ink jet printer (hereinafter, simply referred to as a printer) that includes an ink jet printing head (hereinafter, simply referred to as a printing head) as a liquid ejecting head and printing an image or the like by ejecting and landing liquid drop-shaped ink from a nozzle of the printing head onto a printing medium (ejection target) such as a printing sheet. In recent years, the application of the liquid ejecting apparatus has not been limited to an image forming apparatus, and the liquid ejecting apparatus has been applied to various manufacturing apparatuses. For example, in an apparatus for manufacturing a display such as a liquid crystal display, a plasma display, an organic EL (Electro Luminescence) display, or an FED (Field Emission Display), the liquid ejecting apparatus has been used in order to eject various liquid materials such as a color material or an electrode onto a pixel formation area or an electrode formation area.
The printing head includes a channel unit to which ink is introduced from a liquid storing portion such as an ink cartridge enclosing liquid ink therein and which is provided with a series of liquid channels formed from a reservoir to a nozzle via a pressure chamber or an actuator unit which has a pressure generating element capable of changing the volume of the pressure chamber. In the printing head, since the ink is thickened due to natural evaporation or a pressure loss is caused by bubbles mixed with the ink and absorbing a variation in the pressure, so-called dot skipping may be caused when no ink is ejected from the nozzle or the flight path may be curved. As a result, there is a problem in that ink ejection errors occur in the printing head.
In order to prevent such ink ejection errors, various maintenance processes are performed. For example, a maintenance process is performed, which forcibly removes the thickened ink or bubbles mixed with the ink by performing idle ejection of a liquid droplet (hereinafter, referred to as a flushing) from the nozzle in such a manner that a pressure generating element is driven to cause a variation in the pressure inside the pressure chamber. In order to further reliably discharge the bubbles existing in the liquid channel together with the liquid from the nozzle by the flushing, it is necessary to apply a comparatively large variation in the pressure as large as possible to the bubbles. Therefore, for example, JP-A-2009-73074 discloses a printer capable of generating a maintenance driving pulse having a large variation in the pressure to be applied to the inside of a pressure chamber by resonating a variation in the pressure to be applied to the pressure chamber in accordance with natural oscillation of the liquid generated inside the pressure chamber using the pressure generating element.
However, in recent years, there has been an attempt to make the printer eject a liquid (for example, UV ink (UV curable ink) or the like and hereinafter, referred to as a highly viscous liquid) having viscosity higher than that of a liquid (for example, water-based ink) used so far. That is, in the past, a liquid having a low viscosity of 1 to 5 mPa·s or so was used. However, in recent years, there has been an attempt to make the ink jet printer eject a highly viscous liquid having viscosity of 8 mPa·s or more. In order to obtain a sufficient ejection amount of the highly viscous liquid, it is necessary to eject the liquid after the printing head is heated to thereby decrease the viscosity of the liquid. However, when the temperature of the liquid is increased by heating the printing head, the solubility of the bubbles dissolved in the liquid decreases, and as a result, there is a tendency that the bubbles exceeding the solubility are easily discharged from the liquid into the liquid channel. For this reason, there is a problem in that the performance of discharging the bubbles is degraded even when the flushing process is performed in a high temperature state in which the solubility of the bubbles with respect to the liquid decreases.